When we look at our lives, we tend to break them up into chapters, rather like the seasons of a TV box set. Potential dividers come in many forms, including the dawn of a new year, or the start of a new job. But if those events act as a marker between episodes, it is the decades of our lives that represent the more profound end of one series or season and the start of the next.

According to the psychologists Adam Alter and Hal Hershfield, when we’re on the cusp of one of these boundaries – in other words, when our age ends in a “9”, such as 29, 39, 49 or 59 – we are particularly prone to reflect on the meaning of our lives. If we don’t like what we see, their new results suggest we take drastic action, either fleeing life’s emptiness, or setting ourselves new goals.

The pair began by looking at data from the World Values Survey. Based on answers from 42,063 adults across 100 nations, they found that people with an age ending in 9 (the researchers call these people “9-enders”) were more likely than people of other ages to say that they spent time thinking about the meaning and purpose of their lives.

In another study, participants prompted to imagine and write about how they would feel the night before entering a new decade, tended to say they would think about the meaning of their life more than did other participants who’d been prompted to write about the night before their next birthday, or to write about tomorrow.

At the dawn of a new decade, how does this focus on life’s meaning affect our behaviour? Alter and Hershfield say that for some people it can lead to “maladaptive behaviours”. They looked at data from an online dating website that caters for people who are seeking extramarital affairs. Among over 8 million male users of the site, 9-enders were over-represented by 17.88 per cent relative to what you’d expect if participation were randomly distributed by age. The same was true, though to a lesser extent, for female users of the site.

For some people, the self-reflection triggered by the prospect of entering a new decade is more than they can bear. Alter and Hershfield also examined suicide data collected between 2000 and 2011 by the US Center for Disease Control and Prevention. They found that 9-enders take their own lives with a greater frequency than people whose ages end in any other digit.

It seems the “crisis of meaning” triggered by the prospect of a new decade can also lead people to set themselves new goals. When the researchers looked at data on the Athlinks website, they found that among 500 first-time marathon runners, 9-enders were over-represented by 48 per cent. The same site also contained evidence of 9-enders investing greater effort into their training and performance. Focusing on data from runners in their twenties, thirties and early forties who’d run a marathon at the end of a decade and also in the preceding and following two years, the researchers found that people achieved better times, by an average of 2.3 per cent, when they were aged 29 or 39 than when they were one or two years younger.

The researchers said there’s a growing literature that suggests “although people age continually, the passage of time is more likely to influence their thoughts and actions at some ages than others.” They added: “Here we find that people are significantly more likely to consider whether their lives are meaningful as they approach the start of a new decade.”